Medical devices, such as cardiac pacemakers, cardiac defibrillators, or implantable cardioverter-defibrillators, provide therapeutic electrical stimulation to a heart of a patient via electrodes carried by one or more medical leads and/or electrodes on a housing of the medical device. The electrical stimulation may include signals such as pulses or shocks for pacing, cardioversion or defibrillation. In some cases, a medical device may sense intrinsic depolarizations of the heart, and control delivery of stimulation signals to the heart based on the sensed depolarizations. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation signal or signals may be delivered to restore or maintain a more normal rhythm of the heart. For example, in some cases, an implantable medical device (IMD) or an external medical device may deliver pacing pulses to the heart of the patient upon detecting tachycardia or bradycardia or deliver cardioversion or defibrillation shocks to the heart upon detecting tachycardia or fibrillation.
In general, medical devices detect tachycardia or fibrillation of the atria or ventricles based on the intervals between depolarizations, which are a function of the rate of the depolarizations. Accordingly, in some examples, a medical device detects a ventricular tachycardia of the intervals between ventricular depolarizations are less than a first threshold, and ventricular fibrillation of the intervals between ventricular depolarizations are less than a second threshold. Other features, such as rate variability or electrogram morphology, have been used to classify or distinguish various types of arrhythmias.
In some cases, a fast ventricular depolarization rate may be the result of a sinus tachycardia or an atrial tachyarrhythmia, and is referred to as a supraventricular tachycardia. Delivery of therapy to the ventricles, such as a cardioversion or defibrillation pulse, in response to a supraventricular tachycardia will likely be ineffective and is undesired. Some medical devices distinguish between ventricular and supraventricular tachycardias by comparison of the rates of or intervals between depolarizations of the ventricles and atria. However, such comparisons may not effectively distinguish ventricular tachycardias from supraventricular tachycardias in all cases. For example, where both the atrial and ventricular depolarizations are rapid and have a substantially 1:1 correspondence, the cause may by a supraventricular tachycardia or a ventricular tachycardia with retrograde conduction to the atria.